Rusts are plant diseases caused by pathogenicfungi of the order Pucciniales (previously also known as Uredinales). An estimated 168 rust genera and approximately 7000 species, more than half of which belong to the genus Puccinia, are currently accepted. [1] Rust fungi are highly specialized parasites with several unique features. A single species may produce up to five morphologically and cytologically distinct spore-producing structures viz., spermagonia, aecia, uredinia, telia, and basidia in successive stages of reproduction. Unlike other plant pathogens, rust usually affects healthy and vigorously growing plants, so the infection is limited to plant parts, such as leaves, petioles, tender shoots, stem, fruits, etc.[1] Perennial systemic infection may cause deformities such as growth retardation, witches brooms, stem canker, hypertrophy of the affected tissues or formation of galls. Plants with severe rust infection may appear stunted, chlorotic (yellowed), or otherwise discoloured. Rust sporulates on affected plant parts. [1] Rust is most commonly seen as coloured powder, composed of tiny aeciospores which land on vegetation producing pustules, or uredia, that form on the lower surfaces. During late spring or early summer, yellow orange or brown, hairlike or ligulate structures called telia grow on the leaves or emerge from bark of woody hosts such as Juniperus species. These telia produce teliospores which will germinate into aerial basidiospores, spreading and causing further infection.

The taxonomy of Pucciniales is complex and the darker coloured smuts can be mistaken for rust. Rusts are so named after the reddish rusty looking sori and the disease is usually noticed after rain.

The group is considered one of the most harmful pathogens to agriculture,horticulture and forestry. These fungi are major concerns and limiting factors for successful cultivation of agricultural and forestry crops. White pine blister rust, wheatstem rust, and coffee rust are examples of notoriously damaging, economically important rusts. [1]

II-Urediniospores-repeating dikaryotic vegetative spores. These spores are referred to as the repeating stage because they can cause auto-infection (re-infect the same host from which the spores were borne). These spores are red/orange and are a characteristic sign of rust fungus infection.

III-Teliospores-Diploid spores that produce basidia and are the survival stage of life cycle

IV-Basidiospores-stem from basidia. Haploid spores which infect the alternate host.[2][3] although these are rarely observed outside of the laboratory.

Rust fungi can be categorized by how many types of spores are produced during the life cycle. Fungi that produce all five spores (sometimes excluding pycniospores) are termed macrocyclic. Fungi that lack pycniospores and aeciospores in their life cycle are termed microcyclic and always have an autoecious life cycle. Demicyclic fungi delete the uredial (repeating) stage from the life cycle. Understanding the life cycles of rust fungi allows for proper disease management.[4]

All rusts are obligate parasites, meaning that they require a living host to complete their life cycle. They generally do not kill the host plant but can severely reduce growth and yield.[5]Cereal crops can be devastated in one season and trees that get infected in the main stem within their first five years by the rust Cronartium quercuum often die.[6]

Rust fungi can also be categorized by their life cycle. Three basic types of life cycles are recognized based on the number of spore states as macrocyclic, demicyclic, and microcyclic.[1] The macrocyclic life cycle has all spore states, the demicyclic lacks the uredinial state, and the microcyclic cycle lacks both the aecial and uredinial states, thus possess only spermagonia and telia. Spermagonia may be absent from each type but especially the microcyclic life cycle. In macrocyclic and demicyclic life cycles, the rust may be either host alternating (heteroecious), i.e., the aecial state is on one kind of plant but the telial state on a different and unrelated plant, or non-host alternating (autoecious), i.e., the aecial and telial states on the same kind of plant. [1]Heteroecious rust fungi require two unrelated hosts to complete their life cycle, with the primary host being infected by aeciospores and the alternate host being infected with basidiospores. This can be contrasted with an autoecious fungus which can complete its life cycle on a single host species.[7]

There are definite patterns of relationship with host plant groups and the rust fungi that parasitize them. Some genera of rust fungi, especially Puccinia and Uromyces, comprise species that are capable of parasitizing plants of many families. But many genera appear to be rather definitely restricted to certain plants. Host restriction may, in heteroecious species, apply to both phases of life cycle or to only one phase. [1]

The fungi produce asexualspores which disperse by wind, water or by insect vectors[8] spreading the infection.

Rust fungi are biotrophs taking nutrients from living cells. When airborne spores settle on a plant, weak hydrophobic interactions are formed with the cutin on the plant cell surface, securing it. By a process not fully understood, the production of mucous like substances called 'adhesins', initially stick the spore to the plant surface.[9]

Once attached, the spore germinates by growing a germ tube and eventually locates a stoma by a touch responsive process known as thigmotropism. This involves growing towards a ridge between the epidermal cells, followed by a perpendicular growth which end at the stoma.[10]

Rust hypha attacking stoma (1600x magnification)

Inside the stoma, the hyphae tips flatten out to form structures known as appressoria that lock to the cell walls.[11] It is thought that the whole process is mediated by a mechano-sensitive calciumion channel, located within the germ tube tip, which produces electric currents that stretch the cell membranes, changing gene expression and forming the appresorium.[12]

Then a peg grows into the plant's mesophyll cells. The peg produces specialised hyphal tips, known as haustoria. These spread around the plant cells without invading the membranes. The plant cell membranes invaginate around the main haustorial body forming a space known as the extra-haustorial matrix. An iron and phosphorus rich neck band bridges the plant and fungal membranes in the space between the cells for water flow, known as the apoplast, thus preventing the nutrients reaching the plant's cells. The haustorium contains amino acid and hexosesugar transporters and H+-ATPases which are used for active transport of nutrients from the plant nourishing the fungus.[13] It continues growing until spore growth occurs. The process repeats every 10 – 14 days, producing numerous spores, carried by wind to new hosts.

The control methods of rust fungus diseases depend largely on the life cycle of the particular pathogen. The following are examples of disease management plans used to control macrocyclic and demicyclic diseases:-

Macrocyclic Disease: Developing a management plan for this type of disease depends largely on whether the repeating stage (urediniospores) occur on the economically important host plant or the alternate host. For example, the repeating stage in white pine blister rust disease does not occur on white pines but on the alternate host, Ribes spp. During August and September Ribes spp. give rise to teliospores which infect white pines. Removal of the alternate host disrupts the life cycle of the rust fungi Cronartium ribicola, preventing the formation of basidiospores which infect the primary host. Although spores from white pines cannot infect other white pines, survival spores may overwinter on infected pines and reinfect Ribes spp. the following season. Infected tissue is removed from white pines and strict quarantines of Ribes spp. are maintained in high risk areas.[18][19]

Puccinia graminis is a macrocyclic heteroecious fungus that causes wheat stem rust disease. The repeating stage in this fungus occurs on wheat and not the alternate host, barberry. The repeating stage allows the disease to persist in wheat even though the alternate host may be removed. Planting resistant crops is the ideal form of disease prevention, however, mutations can give rise to new strains of fungi that can overcome plant resistance. Although the disease cannot be stopped by removal of the alternate host, the life cycle is disrupted and the rate of mutation is decreased because of reduced genetic recombination. This allows resistance bred crops to remain effective for a longer period of time.[20][21]

Demicyclic Disease: Because there is no repeating stage in the life cycle of demicyclic fungi, removal of the primary or the alternate host will disrupt the disease cycle. This method, however, is not highly effective in managing all demicyclic diseases. Cedar-apple rust disease, for example, can persist despite removal of one of the hosts since spores can be disseminated from long distances. The severity of Cedar-apple rust disease can be managed by removal of basidiospore producing galls from junipers or the application of protective fungicides to junipers.[22]

Rust is very hard to treat. Fungicides such as Mancozeb or Triforine may help but may never eradicate the disease. Some organic preventative solutions are available and sulphur powder is known to stop germination. High standards of hygiene and good soil drainage and careful watering may minimise problems. Any appearance of rust must be immediately dealt with by removing and burning all affected leaves. Composting, or leaving infected vegetation on the ground will spread the disease.

In large plantations in USA, fungicides are applied by air. The process is expensive and fungicide application is best reserved for seasons when foliar diseases are severe. Research indicates, the higher the foliar disease severity, the greater the return from the use of fungicides.[23] There are a variety of preventative methods that can also be employed.

Symptoms of rust disease are correlated to relatively high moisture. The avoidance of overhead watering at night, using drip irrigation, reducing crop density, and using fans to circulate air flow will lower the relative moisture and decrease the severity of rust infection.

The use of rust resistant plants

Crop rotation can break the disease cycle because many rusts are host specific.

Inspect all imported plants and cuttings for symptoms. It is important to continuously observe these plants because rust diseases have a latent period (plant has the disease but shows no symptoms).

Many crops, such as wheat, are replanted with disease-free seed.[24][25]